MOS field-effect transistors with very thin gate dielectrics made from silicon dioxide may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high-k dielectric materials, instead of silicon dioxide, can reduce gate leakage. Because, however, such a dielectric may not be compatible with polysilicon, it may be desirable to use metal gate electrodes in devices that include high-k gate dielectrics.
The optimal workfunction for a metal gate electrode will differ depending upon whether it is used to form an NMOS transistor or a PMOS transistor. For that reason, when the same material is used to make metal gate electrodes for NMOS and PMOS transistors, the gate electrodes cannot demonstrate the desired workfunction for both types of devices. It may be possible to address this problem by forming the NMOS transistor's metal gate electrode from a first material and the PMOS transistor's metal gate electrode from a second material. The first material may ensure an acceptable workfunction for the NMOS gate electrode, while the second material may ensure an acceptable worktunction for the PMOS gate electrode. Processes for forming such dual metal gate devices may, however, be complex and expensive.
Accordingly, there is a need for an improved process for making a semiconductor device that includes a metal gate electrode. There is a need for a relatively inexpensive and uncomplicated process for making a device with metal gate electrodes that have optimal workfunctions for both NMOS and PMOS transistors. The method of the present invention provides such a process.
Features shown in these figures are not intended to be drawn to scale.